I just ran across this idea myself, but I guess it wasn't original. At any rate, I have a SketchUp file demonstrating this and some sample screen shots.

Plan View
Perspective View
Shadows in Section Cut

Seems it would be a pretty easy ruby script.

User starts with section cut selected. Ruby copies all objects in active group to a new group. Changes materials Pastes section cut in place reverses section cut direction.

If any geometry is changed the script has to be re-run.